Dispersion managed fiber stretcher for high-energy short pulse femotosecond fiber laser system

ABSTRACT

A fiber Chirped Pulse Amplification (CPA) laser system that includes a fiber mode-locking oscillator for generating a seed laser for projecting to a stretcher for generated a pulse-stretched laser for projecting to a multiple stage amplifier. The multiple stage amplifier further amplifying said laser for projecting to a compressor for compressing said laser to generate an output laser of an original pulse width. In this invention, pulse stretcher is implemented with a special dispersion management fiber that has a flat dispersion or a negative TOD (dispersion slope, or a slope of dispersion versus wavelength).

This Formal application claims a Priority Date of Mar. 6, 2006 benefitfrom a Provisional Patent Applications 60/781,434 filed by the sameInventor of this Application. The disclosures made in 60/781,434 arehereby incorporated by reference in this patent application.

FIELD OF THE INVENTION

The present invention relates generally to apparatuses and methods forproviding fiber laser system. More particularly, this invention relatesa system configuration implemented with pulse stretching management fordispersion compensation for providing a practical approach to provide afemtosecond fiber laser with one mJ level of energy.

BACKGROUND OF THE INVENTION

Even though current technologies of fiber laser have made significantprogress toward achieving a compact and reliable fiber laser systemproviding high quality output laser with ever increasing output energy,however those of ordinary skill in the art are still confronted withtechnical limitations and difficulties. Specifically, in a fiber lasersystem implemented with the Chirped Pulse Amplification (CPA) for shortpulse high power laser amplifier, the fiber laser systems are stilllimited by the technical difficulties that 1 mJ high energy femtosecondfiber laser requires multiple improvements in terms of fiber design,high power amplification, nonlinear effects mitigation, and stretchingand compression operations. There is a first challenge of the nonlineareffects. When the peak power goes up to 100 kW, strong nonlinear effectssuch as self phase modulation (SPM) and stimulated Raman scattering(SRS) cause more serious problems in depleting signal power in the highpower fiber laser, even though a large mode area (LMA) fiber be used toreduce SRS/SPM and increase saturation power. Then, there is anotherchallenge of a third order dispersion (TOD), i.e., the dispersion slope,or the slope of dispersion versus wavelength. Due to a higher stretchingratio involves in the chirped pulse amplification, higher orderdispersion such as TOD has significant impact on the pulse quality andthe pulse faces a challenge to compress efficiently below 200 fs afteramplification. Thus the third order dispersion (TOD) limits thescalability of the laser systems.

There are additional difficulties in power extraction when the Yb-fibersare implemented due to the low extraction of the power output from thefiber. Higher doping concentrations are required for use in the fiberlaser in order to overcome such difficulties. Furthermore, a longcompression stage is required due to the longer stretched pulses at 1 to10 ns pulse-width and that increases the size and costs of such lasersystems. All these challenges require new and improved fiber lasersystems to reliably and practically generate the femtosecond laser at anenergy level substantially near a one-mJ level.

Therefore, a need still exists in the art of fiber laser design andmanufacture to provide a new and improved configuration and method toprovide fiber laser to compensate the dispersion generated in the lasersystem due to the TOD effects such that the above-discussed difficultymay be resolved.

SUMMARY OF THE PRESENT INVENTION

It is therefore an aspect of the present invention to provide a shortpulse fiber laser amplification system by implement a pulse stretcherwith a special dispersion management fiber that has a flat dispersion ora negative dispersion slope such that by managing the dispersions theabove-discussed difficulties as that encountered in the prior art may beresolved.

Specifically, it is an aspect of the present invention that a pulsestretcher is implemented with a fiber of flat dispersion over thespectral range of 1060 nm or a negative dispersion slope over the rangeof 1020-1090 nm by using a depressed cladding structure.

Another aspect of this invention is to implement a stretcher with fibersof different dispersion and dispersion slopes depending on therequirements of managing or compensating the TOD. The fiber implementedin the stretcher may include a negative dispersion slop about twice thatof SM-28 or using a SSMF, corning fiber and dispersion compensationfiber HSDK supplied by OFS Denmark to achiever various dispersions anddispersion slopes in tailing the dispersion for particular requirementsof managing and compensation the TOD or dispersions of higher orders ina laser system.

Briefly, in a preferred embodiment, the present invention discloses afiber Chirped Pulse Amplification (CPA) laser system that includes afiber mode-locking oscillator for generating a seed laser for projectingto a stretcher for generated a pulse-stretched laser for projecting to amultiple stage amplifier. The multiple stage amplifier furtheramplifying said laser for projecting to a compressor for compressingsaid laser to generate an output laser of an original pulse width. Inthis invention, pulse stretcher is implemented with a special dispersionmanagement fiber that has a flat dispersion or a negative TOD(dispersion slope, or a slope of dispersion versus wavelength).

In a preferred embodiment, this invention further discloses a method forconfiguring a fiber Chirped Pulse Amplification (CPA) laser systemincludes a step of generating a seed laser from fiber mode-lockingoscillator for projecting to a fiber stretcher for stretching a pulsewidth of the laser wherein the stretcher further comprising a specialdispersion management fiber that has a flat dispersion or a negativeslope of dispersion versus wavelength.

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the preferred embodiment,which is illustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram for showing a fiber laser systemimplemented with pulse stretcher having a special dispersion managementfiber that has a flat dispersion or a negative dispersion of thisinvention.

FIG. 2 is a fiber dispersion index profile over range of laserwavelengths for different types of fibers that may be implemented in thepulse stretcher of this invention for dispersion management andcompensation.

FIG. 3 is a diagram of TOD grating as function of groove density.

FIG. 4 is a schematic diagram for illustrating the TOD compensation.

FIG. 5 shows curves for illustrating the pulse-shape improvement withTOD compensation accomplished by the new fibers implemented in thisinvention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1 for a schematic diagram of a short pulse high-energyfiber laser system that includes a laser seed 15 having an oscillatorfor generating a fiber-based mode-locking laser with original pulseduration. The laser project from the oscillator of the seed laser 15 isprojected into a laser stretcher 20 to stretch the laser pulse of thisinvention. The stretcher 20 chirps laser pulse with stretched pulsewidth is projected into a series of laser amplifiers 25 to amplify thelaser into higher energy. The amplified laser is then projected into apulse compressor 30 to re-compress the pulse width of the laser tooutput a laser with original pulse width. The pulse stretcher isimplemented with a special dispersion management fiber that has a flatdispersion or a negative slope of dispersion versus wavelength.

FIG. 2 is a diagram for illustrating the fiber dispersion index profileat different wavelength that may be implemented in the stretcher 20 ofFIG. 1. In an exemplary embodiment, it is possible that a fiber with aflat dispersion may be implemented as a new fiber for the stretcher at a1060 nm spectral band. Alternately, the stretcher may be implementedwith a fiber of a negative dispersion slope over the range of 1020-1090nm by using a depressed cladding structure. Depending on the laserdesign in managing/compensating TOD with the nonlinear effects throughthe self phase modulation (SPM), fibers with various dispersion anddispersion slope can be designed. According to FIG. 2, a fiber as afirst new fiber embodiment of this invention, i.e., a new fiber 1, has aflat dispersion over the spectral range of 1060 nm or another fiber as asecond new fiber embodiment of this invention, e.g., a new fiber 2, hasa negative dispersion slope, about twice that of SM 28, may beimplemented. Moreover, due to the negative dispersion properties, thefiber with negative dispersion slope can be used with other types ofcommercial fibers such as SM 28 (SSMF, corning) and dispersioncompensation fiber HSDK (OFS, Denmark) to achieve various dispersionsand dispersion slopes in tailoring the dispersion of the fiber laser.

Referring to FIG. 3 as an example for illustrating the reason formanaging the dispersion and slope to resolve the TOD effects thattypically generated from a compressor implemented with a grating pair.Compared with a SM28 fiber, the TOD is about at least twice larger thanthat of SM 28. This amount of TOD must be compensated by a fiber with anegative dispersion slope (TOD) such as the fiber with a negativedispersion slope twice that of SM 28.

A dispersion slope is defined as the dispersion change divided by thewavelength change (or differential change of the dispersion over a givenwavelength change). To show how to compensate the TOD of one component,e.g. fiber or grating, FIG. 4 illustrates the improvements asillustrated by line “(1+2) compensated slope”, accomplished by thecompensation when a laser system is implemented with the specialdispersion management fiber that either has a negative slope ofdispersion indicated as line 2 in the FIG. 4.

An experiment has been done for a 10 micro-Joule high-energy fs fiberlaser system operating at 1030 nm in comparison between SM 28 fiberstretcher and a new fiber according to the second new fiber embodimentof this invention, e.g., a new fiber 2 shown in FIG. 2 that has anegative dispersion slope about twice that of SM 28. It shows that about40% pulse reduction has been improved. FIG. 5 gives the pulse widthresults.

This invention discloses an ultra-fast fiber laser system that includesa dispersion managed fiber stretcher in order to compensate a TOD(dispersion slope, or a slope of dispersion versus wavelength) of agrating compressor or in a regular fiber. In an exemplary embodiment,the dispersion managed fiber stretcher comprising fibers having arefractive index profile different from a conventional fiber., forexample a depressed cladding structure. In another exemplary embodiment,the dispersion managed fiber stretcher comprising a fiber of a flatdispersion. In another exemplary embodiment, the dispersion managedfiber stretcher comprising a fiber of a negative TOD (dispersion slope,or a slope of dispersion versus wavelength). In another exemplaryembodiment, the dispersion managed fiber stretcher comprising a fiber ofa design for managing/compensating a TOD with nonlinear effects througha self phase modulation (SPM) for providing different dispersions anddispersion slopes according to the TOD in the ultra-fast fiber lasersystem. In another exemplary embodiment, the dispersion managed fiberstretcher comprising a fiber of a negative TOD (dispersion slope, or aslope of dispersion versus wavelength) about twice of a SM 28 fiber. Inanother exemplary embodiment, the ultra-fast laser system operates at 1μm region (1030-1100 nm). In another exemplary embodiment, theultra-fast laser system generates a laser with a pulse width from 10 psto 10 fs. In another exemplary embodiment, the dispersion managed fiberstretcher includes a fiber of a SSMF Corning fiber. In another exemplaryembodiment, the dispersion managed fiber stretcher includes a fiber of aHSDK fiber.

Therefore, according to above descriptions and drawings, this inventiondiscloses a fiber laser system that includes a dispersion-managedstretcher to chirp a laser pulse wherein the dispersion-managedstretcher further comprising a fiber having a flat dispersion.Furthermore, in an alternate embodiment, this invention discloses afiber laser system that includes a dispersion-managed stretcher to chirpa laser pulse wherein the dispersion-managed stretcher furthercomprising a fiber having a negative TOD (dispersion slope, or a slopeof dispersion versus wavelength). This invention also discloses a methodfor generating a laser by a fiber laser system that includes a step ofchirping a laser pulse by a dispersion managed stretcher by implementinga fiber with a flat dispersion in the dispersion managed stretcher. Inan alternate embodiment, this invention also discloses a method forgenerating a laser by a fiber laser system that includes a step ofchirping a laser pulse by a dispersion-managed stretcher by implementinga fiber with a negative TOD (dispersion slope, or a slope of dispersionversus wavelength) in the dispersion managed stretcher.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alternationsand modifications will no doubt become apparent to those skilled in theart after reading the above disclosure. Accordingly, it is intended thatthe appended claims be interpreted as covering all alternations andmodifications as fall within the true spirit and scope of the invention.

1-47. (canceled)
 48. A fiber-based Chirped Pulse Amplification (CPA)laser system, comprising: a seed laser configured to generate a firstlaser pulses; a fiber-based stretcher configured to produce a stretchedlaser pulse longer than the first laser pulse, wherein the fiber-basedstretcher has a first optical dispersion; a fiber-based amplifierconfigured to produce an amplified pulse in response to the stretchedlaser pulse; and a compressor configured to compress the amplified pulseto produce an output laser pulse shorter than the amplified pulse,wherein the compressor has a second optical dispersion thatsubstantially compensates the first optical dispersion such that thefiber-based stretcher and the compressor in combination produce acombined optical dispersion that is substantially unchanged in apredetermined wavelength range.
 49. The fiber-based CPA laser system ofclaim 48, wherein the first optical dispersion and the second opticaldispersion vary oppositely as functions of wavelength.
 50. Thefiber-based CPA laser system of claim 48, wherein the first opticaldispersion decreases as a function of wavelength in a predeterminedwavelength range, wherein the second optical dispersion increases as afunction of wavelength in the predetermined wavelength range.
 51. Thefiber-based CPA laser system of claim 48, wherein the predeterminedwavelength range is from about 1000 nm to about 1100 nm.
 52. Thefiber-based CPA laser system of claim 48, wherein the output laser pulsehas a pulse width ranging from about 10 femtoseconds to about 10picoseconds.
 53. The fiber-based CPA laser system of claim 48, whereinthe output laser pulse has a pulse energy between 1 nJ and 1 mJ.
 54. Thefiber-based CPA laser system of claim 48, wherein the seed lasercomprises a fiber-based mode-locking oscillator configured to generatethe first laser pulse.
 55. The fiber-based CPA laser system of claim 48,wherein the compressor comprises an optical fiber, a fiber grating, or abulk grating pair.
 56. The fiber-based CPA laser system of claim 48,wherein the fiber-based stretcher comprises a depressed claddingstructure.
 57. A fiber-based Chirped Pulse Amplification (CPA) lasersystem, comprising: a seed laser configured to generate a first laserpulses; a fiber-based stretcher configured to produce a stretched laserpulse longer than the first laser pulse, wherein the fiber-basedstretcher has a first third order dispersion (TOD); a fiber-basedamplifier configured to produce an amplified pulse in response to thestretched laser pulse; and a compressor configured to compress theamplified pulse to produce an output laser pulse shorter than theamplified pulse, wherein the compressor has a second TOD thatsubstantially compensates the first TOD such that the fiber-basedstretcher and the compressor in combination produce a combined TOD thatis substantially unchanged in a predetermined wavelength range.
 58. Thefiber-based CPA laser system of claim 57, wherein the first TOD and thesecond TOD vary oppositely as functions of wavelength.
 59. Thefiber-based CPA laser system of claim 57, wherein the first TODdecreases as a function of wavelength in a predetermined wavelengthrange, wherein the second TOD increases as a function of wavelength inthe predetermined wavelength range.
 60. The fiber-based CPA laser systemof claim 57, wherein the predetermined wavelength range is from about1000 nm to about 1100 nm.
 61. The fiber-based CPA laser system of claim57, wherein the output laser pulse has a pulse width ranging from about10 femtoseconds to about 10 picoseconds.
 62. The fiber-based CPA lasersystem of claim 57, wherein the output laser pulse has a pulse energybetween 1 nJ and 1 mJ.
 63. The fiber-based CPA laser system of claim 57,wherein the predetermined wavelength range is from about 1000 nm toabout 1100 nm.
 64. The fiber-based CPA laser system of claim 57, whereinthe compressor comprises an optical fiber, a fiber grating, or a bulkgrating pair.
 65. The fiber-based CPA laser system of claim 57, whereinthe fiber-based stretcher comprises a depressed cladding structure. 66.A method for producing a laser pulse, comprising: generating a firstlaser pulses using a fiber-based mode-locking oscillator; producing astretched laser pulse longer than the first laser pulse using afiber-based stretcher that has a first third order dispersion (TOD);producing an amplified pulse using a fiber-based amplifier in responseto the stretched laser pulse; and using a compressor to compress theamplified pulse to produce an output laser pulse shorter than theamplified pulse, wherein the compressor has a second TOD thatsubstantially compensates the first TOD such that the fiber-basedstretcher and the compressor in combination produce a combined TOD thatis substantially unchanged in a predetermined wavelength range.
 67. Themethod of claim 66, wherein the first TOD and the second TOD varyoppositely as functions of wavelength.
 68. The method of claim 66,wherein the first TOD decreases as a function of wavelength in apredetermined wavelength range, wherein the second TOD increases as afunction of wavelength in the predetermined wavelength range.
 69. Themethod of claim 66, wherein the output laser pulse has a pulse widthranging from about 10 femtoseconds to about 10 picoseconds.
 70. Themethod of claim 66, wherein the output laser pulse has a pulse energybetween 1 nJ and 1 mJ.
 71. The method of claim 66, wherein thepredetermined wavelength range is from about 1000 nm to about 1100 nm.